Germany has closed down the glass and steel dome and rooftop terrace of the famous Reichstag parliament building to visitors over concerns of a possible terror attack.

The building, which attracts thousands of tourists daily for its panoramic views of the city, will be shut until further notice, parliamentary spokeswoman Birgit Landskron said.

The move came after Der Spiegel magazine reported at the weekend that an unidentified informer told German authorities that al Qaida is planning a possible attack early next year on the parliament building in downtown Berlin.

Joerg Ziercke, head of Germany's Federal Criminal Police Office, called the report "highly speculative". "There are no indications of specific places, people or moments, he added.

A spokesman for the federal police, speaking on departmental policy of anonymity, said he could not comment on specifics but that the situation had not changed since Mr Ziercke's comments.

Germans have been on edge since interior minister Thomas de Maiziere raised the country's terrorist threat level last week. He cited an increased threat from Islamic extremists, saying that Germany had received a tip from an unspecified country about a suspected attack planned for the end of November.

The city of Berlin's top security official, Ehrhart Koerting, told a parliamentary security subcommittee that in light of the terrorist warnings, he had decided to strengthen security at the Reichstag building.

In addition to shutting down the cupola, the entire area around the building was fenced off, with a heavy presence of Berlin and federal police standing guard.

Mr Koerting told lawmakers there were "many uncertainties". He was quoted by the news agency DAPD as saying the situation "gives us cause for concern, but no cause for hysteria".